A problem arises in the initialization of apparatus which includes a large input filter capacitance and in the replacement of fuses serving such circuitry due to the large inrush of current to charge the filter capacitance. Such large inrush currents create undesirable variations in the supply voltage and the possibilities of blowing fuses or opening circuit breakers and excessively eroding fuse holders.
In the past these problems have been attacked by utilization of a resistance or the series combination of a resistance and an inductance in the charging path of the capacitance. A prior art arrangement comprises a series combination of a resistance and a switch connected across a fuse holder. The fuse holder is connected between a source of potential and apparatus having an input filter capacitance to be charged. The switch is manually closed such that current will initially flow through the resistance to the apparatus. This current will charge the capacitor to a voltage level which depends on the current drawn by the apparatus and the resistance used. After the capacitance has been thus partially charged, a fuse is inserted into the fuse holder and the switch is manually opened.
A problem in the prior art arises in the selection of a resistance. If a resistance is selected to significantly reduce the power supply voltage variations, the capacitance will not be charged to a sufficiently large voltage to insure not blowing a fuse. If a resistance is selected to charge the capacitance to a voltage level which will insure not blowing a fuse, the inrush current will be sufficiently large to induce significant power supply voltage variations. Thus a compromise must be made.